p54: only unregister ieee80211_hw when it has been registered

p54_unregister_common may now be called by the backend
driver's remove routine, even if the ieee80211_hw device
struct was never successfully registered.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>